Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
3rd generation systems have recently been rolled out in many areas to further enhance the communication services provided to mobile users. One such system is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876. The core network of UMTS is built on the use of SGSNs and GGSNs thereby providing commonality with GPRS.
As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the mobile station moves towards a base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the mobile station moves further into the new cell, it continues to be supported by the new base station. This is known as a handover or handoff of a mobile station between cells.
As an increasing number of communication systems, such as GSM, UMTS, Wireless Local Area Network (WLAN etc), are introduced mobile stations are being deployed which are capable of accessing different communication systems. Indeed, the variety of systems has led to a design philosophy where interworking between different communication systems and air interface standards has become a priority. Indeed, the different radio access standards and technologies are often viewed as different radio access networks of a combined hybrid communication system having a common or interfaced core network. These different radio access networks or air interface standards are often referred to as different access layers.
In order to achieve efficient interworking between different radio access technologies, handover of communication services between these has become increasingly important. For example, a mobile station may comprise both GSM and UMTS functionality and may handover an ongoing call from a GSM RAN to a UMTS RAN or vice versa. Such interlayer handover may provide enhanced communication services and an improved user experience. However, the interlayer handover requires complex management procedures. Also, the different RANs have different characteristics and the characteristics of the service provided to the mobile station are typically dependent on the RAN currently supporting the service.
Thus, in mobile communications systems, different access layers may have different capabilities and may support some services better than others. To deal with this issue, the 3rd Generation Partnership Project (3GPP) Technical Specifications provide for a Service Handover mechanism between GSM and UMTS. The handover mechanism seeks to define an access layer preference for a communication service such that for a given service it is defined which access layer the service is preferably supported by. Hence, the service is supported by the preferred access layer if this is possible and the alternative layer is only used if the preferred layer cannot support the service.
Specifically, for GSM and UMTS, handovers are managed within the serving RAN based on the conditions of the individual mobile station. However, the service itself is set up and controlled by the core network and the RAN has only information of the characteristics of the radio bearers supporting the service but not of the characteristics of the service itself.
Accordingly, 3GPP have introduced an access layer preference in the Handover Service Information Element which is an information element communicated from the Core Network to the RAN. For a GSM RAN, the access layer preference has the following states for a given service:                Unset—no preference is indicated.        There is no preference for the service for any access layer.        Should handover to UMTS.        If possible the service should be handed over to the UMTS access layer. The GSM RAN hands the service over to UMTS if the radio conditions allow this.        Should not handover to UMTS        If possible the service should be continued to be supported by the GSM RAN. The service is only handed over to UMTS if the radio conditions require this to continue to support the service.        Shall not handover to UMTS.        The service must be supported by the GSM RAN or dropped. The service cannot be handed over to UMTS.        
The Handover Service Information Element sent from the core network to the UMTS RAN comprises the following settings:                Unset—no preference is indicated.        There is no preference for the service for any access layer.        Should handover to GSM.        If possible the service should be handed over to the GSM access layer. The UMTS RAN hands the service over to GSM if the radio conditions allow this.        Should not handover to GSM        If possible the service should be continued to be supported by the UMTS RAN. The service is only handed over to GSM if the radio conditions require this to continue to support the service.        Shall not handover to GSM.        The service must be supported by the UMTS RAN or dropped. The service cannot be handed over to GSM.        
Thus, the Handover Service Information Element allows the RANs to perform handovers such that a given service is supported by the preferred access layer whenever possible. However, although this may provide advantages in many situations, it does not provide optimal performance in all conditions or for all services and in particular it may result in large handover overheads, reduced user quality and increased resource usage.
Hence, an improved handover would be advantageous in many scenarios.